| Year | Value | | 2021 | 4.3532035454618 | | 2020 | 4.2646384677417 | | 2019 | 4.3107034785218 | | 2018 | 4.3153880558892 | | 2017 | 4.2951997581866 | | 2016 | 4.2975420468703 | | 2015 | 4.269880732891 | | 2014 | 4.2419963437989 | | 2013 | 4.2142234922632 | | 2012 | 4.1864506407275 | | 2011 | 4.1586777891918 | | 2010 | 4.2035158868518 | | 2009 | 4.2480193718428 | | 2008 | 4.2925228568337 | | 2007 | 4.3370263418247 | | 2006 | 4.3815298268156 | | 2005 | 4.4186718330863 | | 2004 | 4.4558138393569 | | 2003 | 4.4929558456276 | | 2002 | 4.5300978518982 | | 2001 | 4.5672398581688 | | 2000 | 4.5698052219653 | | 1999 | 4.5722590482054 | | 1998 | 4.5747128744455 | | 1997 | 4.5771667006856 | | 1996 | 4.5796205269257 | | 1995 | 4.5853089423005 | | 1994 | 4.5911088952317 | | 1993 | 4.5969088481628 | | 1992 | 4.602708801094 | | 1991 | 4.6084429357131 | | 1990 | 4.6139740942872 | | 1989 | 4.6198855847747 | | 1988 | 4.6257970752622 | | 1987 | 4.6317085657497 | | 1986 | 4.6376200562372 | | 1985 | 4.6152010074072 | | 1984 | 4.5928934961336 | | 1983 | 4.5705859848599 | | 1982 | 4.5482784735862 | | 1981 | 4.5259709623126 | | 1980 | 4.4945173714167 | | 1979 | 4.4632868556336 | | 1978 | 4.4320563398505 | | 1977 | 4.4008258240673 | | 1976 | 4.3695953082842 | | 1975 | 4.35621080152 | | 1974 | 4.3426032196431 | | 1973 | 4.3289956377662 | | 1972 | 4.3153880558892 | | 1971 | 4.3017804740123 | | 1970 | 4.294418995292 | | 1969 | 4.286834441459 | | 1968 | 4.2792498876259 | | 1967 | 4.2716653337929 | | 1966 | 4.2640807799598 | | 1965 | 4.228388761922 | | 1964 | 4.1921390561023 | | 1963 | 4.1558893502826 | | 1962 | 4.1196396444629 | | 1961 | 4.0833899386432 |
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